lec5

Switching and Forwarding: 

Switching and Forwarding Outline Store-and-Forward Switches Bridges and Extended LANs Cell Switching Segmentation and Reassembly

Scalable Networks : 

Scalable Networks Switch forwards packets from input port to output port port selected based on address in packet header Advantages cover large geographic area (tolerate latency) support large numbers of hosts (scalable bandwidth)

Source Routing: 

Source Routing

Virtual Circuit Switching: 

Virtual Circuit Switching Explicit connection setup (and tear-down) phase Subsequence packets follow same circuit Sometimes called connection-oriented model Analogy: phone call Each switch maintains a VC table

Datagram Switching: 

Datagram Switching No connection setup phase Each packet forwarded independently Sometimes called connectionless model Analogy: postal system Each switch maintains a forwarding (routing) table

Example Tables: 

Example Tables Circuit Table (switch 1, port 2) Forwarding Table (switch 1)

Virtual Circuit Model: 

Virtual Circuit Model Typically wait full RTT for connection setup before sending first data packet. While the connection request contains the full address for destination, each data packet contains only a small identifier, making the per-packet header overhead small. If a switch or a link in a connection fails, the connection is broken and a new one needs to be established. Connection setup provides an opportunity to reserve resources.

Datagram Model: 

Datagram Model There is no round trip delay waiting for connection setup; a host can send data as soon as it is ready. Source host has no way of knowing if the network is capable of delivering a packet or if the destination host is even up. Since packets are treated independently, it is possible to route around link and node failures. Since every packet must carry the full address of the destination, the overhead per packet is higher than for the connection-oriented model.

Bridges and Extended LANs: 

Bridges and Extended LANs LANs have physical limitations (e.g., 2500m) Connect two or more LANs with a bridge accept and forward strategy level 2 connection (does not add packet header) Ethernet Switch = Bridge on Steroids

Learning Bridges : 

Learning Bridges Do not forward when unnecessary Maintain forwarding table Host Port A 1 B 1 C 1 X 2 Y 2 Z 2 Learn table entries based on source address Table is an optimization; need not be complete Always forward broadcast frames

Spanning Tree Algorithm : 

Spanning Tree Algorithm Problem: loops Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 802.1 specification

Algorithm Overview : 

Algorithm Overview Each bridge has unique id (e.g., B1, B2, B3) Select bridge with smallest id as root Select bridge on each LAN closest to root as designated bridge (use id to break ties) B3 A C E D B2 B5 B B7 K F H B4 J B1 B6 G I Each bridge forwards frames over each LAN for which it is the designated bridge

Algorithm Details: 

Algorithm Details Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be root bridge distance (hops) from sending bridge to root bridge Each bridge records current best configuration message for each port Initially, each bridge believes it is the root

Algorithm Detail (cont): 

Algorithm Detail (cont) When learn not root, stop generating config messages in steady state, only root generates configuration messages When learn not designated bridge, stop forwarding config messages in steady state, only designated bridges forward config messages Root continues to periodically send config messages If any bridge does not receive config message after a period of time, it starts generating config messages claiming to be the root

Broadcast and Multicast: 

Broadcast and Multicast Forward all broadcast/multicast frames current practice Learn when no group members downstream Accomplished by having each member of group G send a frame to bridge multicast address with G in source field

Limitations of Bridges: 

Limitations of Bridges Do not scale spanning tree algorithm does not scale broadcast does not scale Do not accommodate heterogeneity Caution: beware of transparency

Cell Switching (ATM): 

Cell Switching (ATM) Connection-oriented packet-switched network Used in both WAN and LAN settings Signaling (connection setup) Protocol: Q.2931 Specified by ATM forum Packets are called cells 5-byte header + 48-byte payload Commonly transmitted over SONET other physical layers possible

Variable vs Fixed-Length Packets: 

Variable vs Fixed-Length Packets No Optimal Length if small: high header-to-data overhead if large: low utilization for small messages Fixed-Length Easier to Switch in Hardware simpler enables parallelism

Big vs Small Packets: 

Big vs Small Packets Small Improves Queue behavior finer-grained preemption point for scheduling link maximum packet = 4KB link speed = 100Mbps transmission time = 4096 x 8/100 = 327.68us high priority packet may sit in the queue 327.68us in contrast, 53 x 8/100 = 4.24us for ATM near cut-through behavior two 4KB packets arrive at same time link idle for 327.68us while both arrive at end of 327.68us, still have 8KB to transmit in contrast, can transmit first cell after 4.24us at end of 327.68us, just over 4KB left in queue

Big vs Small (cont): 

Big vs Small (cont) Small Improves Latency (for voice) voice digitally encoded at 64KBps (8-bit samples at 8KHz) need full cell’s worth of samples before sending cell example: 1000-byte cells implies 125ms per cell (too long) smaller latency implies no need for echo cancellers ATM Compromise: 48 bytes = (32+64)/2

Cell Format: 

Cell Format User-Network Interface (UNI) host-to-switch format GFC: Generic Flow Control (still being defined) VCI: Virtual Circuit Identifier VPI: Virtual Path Identifier Type: management, congestion control, AAL5 (later) CLPL Cell Loss Priority HEC: Header Error Check (CRC-8) Network-Network Interface (NNI) switch-to-switch format GFC becomes part of VPI field

Segmentation and Reassembly : 

Segmentation and Reassembly ATM Adaptation Layer (AAL) AAL 1 and 2 designed for applications that need guaranteed rate (e.g., voice, video) AAL 3/4 designed for packet data AAL 5 is an alternative standard for packet data AAL A TM AAL A TM … …

AAL 3/4: 

AAL 3/4 Convergence Sublayer Protocol Data Unit (CS-PDU) CPI: commerce part indicator (version field) Btag/Etag:beginning and ending tag BAsize: hint on amount of buffer space to allocate Length: size of whole PDU

Cell Format : 

Cell Format Type BOM: beginning of message COM: continuation of message EOM end of message SEQ: sequence of number MID: message id Length: number of bytes of PDU in this cell

AAL5: 

AAL5 CS-PDU Format pad so trailer always falls at end of ATM cell Length: size of PDU (data only) CRC-32 (detects missing or misordered cells) Cell Format end-of-PDU bit in Type field of ATM header